1. Field of the Invention
The present invention relates to a display device having organic electroluminescent (EL) elements and specifically relates to a display device having organic EL elements that can improve view angle characteristics.
2. Description of the Related Art
Organic compound layers, such as electrodes of organic EL elements, light-emitting layers, and hole-transporting layers, are easily corroded or oxidized by reaction with moisture or oxygen in air. Such corrosion and oxidation significantly develop non-light-emitting portions, so-called dark spots, and cause characteristic degradation of the organic EL elements as time passes. Factors causing dark spots will now be described.
Usually, the surfaces of the organic EL elements are covered with a protective layer for preventing the organic compound layer from being brought into contact with moisture or oxygen in air. However, pinholes may be partially formed in the protective layer due to particles such as etching residues generated in the production process and dust. Then, oxygen and moisture penetrate through the pinholes into the insides of the elements to form dark spots. In order to prevent the penetration of oxygen and moisture, the protective layer may be thick so as to sufficiently cover the particles. However, since a protective layer made of an inorganic material, such as SiN, is formed by CVD or the like, it takes a long time to form the layer to be thick, resulting in a problem of increasing cost. At the same time, a design is proposed where a resin layer, which can be conveniently formed by application or the like, is formed for sufficiently covering the particles and a protective layer of a dense inorganic material is further formed on the resin layer.
Japanese Patent Laid-Open No. 2002-025765 discloses a sealing technique involving forming of a protective layer having a laminate structure composed of silicon nitrided oxide (SiON) layer/organic material layer/silicon nitrided oxide (SiON) layer on the upper electrode of the organic EL element.
Furthermore, in Japanese Patent Laid-Open No. 2005-19148, in order to give satisfactory irradiation distribution and provide natural visibility, a micro optical element having negative refractive power is formed on the upper surface of a covering layer (for example, SiO2) covering the organic EL element.
Incidentally, combination of the protective layer having a laminate structure disclosed in Japanese Patent Laid-Open No. 2002-025765 and the micro optical element (concave microlens) having a negative refractive index shown in Japanese Patent Laid-Open No. 2005-19148 causes the following problems.
That is, when a microlens array made of a resin is disposed on a laminate structure serving as a protective layer, the microlens layer itself is exposed to the exterior atmosphere. Therefore, moisture in the atmosphere readily penetrates into the microlenses to cause changes in the refractive index and the shape due to swelling, resulting in a decrease in lens function.
In order to solve this problem, an outermost protective layer made of an inorganic material such as SiN may be formed on the surfaces of the microlenses. In such a case, the layer structure has five layers including the protective layer composed of three layers and the resin layer constituting the microlenses, which leads to an increase of cost.
In addition, in such a structure, the number of layer interfaces constituting reflecting interfaces is increased, and thereby the quantity of emitted light that is trapped in the element is increased. As a result, the total quantity of light extracted to the outside is decreased, and also thereby luminance in an oblique direction is decreased, which leads to degradation in view angle characteristics.